The invention relates to liquid-level sensors of prismatic construction wherein one or more optical paths into the prism are altered by the presence of a liquid-to-prism interface, as compared to an air-to-prism interface, and wherein light which exits the prism is a function of whether the prism has interface with liquid or with air.
A variety of solid prismatic liquid-level sensors are commercially available, to indicate the presence or absence of liquid at contact with the prism, via reflection of light in the prism or for refraction of light out of the prism, refracted light being dissipated in the liquid. Such prisms are conventionally made of a transparent glass or plastic material. Experience has shown that light injected to such a solid prism tends to scatter and to fill the entire prism; this type of prism is very sensitive to adherent liquid drops and condensation, on operative facets of the prism. Moreover, such solid prisms are not extremely accurate since the sensitive area of the prism (i.e., where refraction-sensing occurs) may vary from prism to prism.
Accompanying FIGS. 1A, 1B and 1C of the drawings are simple diagrams to illustrate problems of known solid-type liquid-level prism sensors of the character indicated. The prism is a solid cone having a 90-degree apex angle and having a flat circular base 10 (FIG. 1A) with two ports 11, 12 at equal but opposite offset from the cone axis, respectively for passage of input light at port 11 into the prism and for passage of any exiting light at port 12 out of the prism, i.e., for any light which seeks to exit at port 12. The light may be supplied to port 11 by any convenient source such as a light-emitting diode, with or without an optical fiber port connection.
In the case of prism exposure to air (FIG. 1B), input light encounters some divergence of limiting rays, which are internally reflected twice before emerging exit from the prism. And due to the noted divergence on prism entry, there is a relatively wide spread of locations at which internally reflected light will exit the prism. The wider this spread, the more inefficient is the ability to obtain a proper photosensitive response to the exiting light.
In the case of prism exposure to or immersion in a liquid, such as water or a fuel (FIG. 1C), input light is refracted from the prism/liquid interface and is absorbed (dissipated) in the liquid. This circumstance results in little or no internally reflected light available to whatever photosensitive device may be employed for response to exiting light. Thus, the decrease in detected exit light response is an indicator of detected liquid at the level at which the prism has been mounted.